According to FGB, it seems like cúram is more 'care and responsibility', while the other is more 'care and attention'. So most likely they're interchangeable.

That's an issue of English, not Irish. When you are talking about a definite subject in English, you normally say something like "the apple is in the bag", but if it's an indefinite subject, you usually say "there is an apple in the bag". ("an apple is in the bag" isn't necessarily wrong, it's just sounds a bit weird).

In Irish, the structure doesn't change - "tá an t-úll sa mhála" (the apple is in the bag), "tá úll sa mhála" ("there is an apple in the bag")